A high voltage transistor is very large in size due to the necessity of a long gate length, a low concentration diffusion region as a drift region, and others, as compared to a low voltage transistor.
To date, a variety of techniques have been proposed to scale down a large-size high voltage transistor. Among them, Japanese Patent Laid-Open Publication No. Hei 8 (1996)-97411 has proposed a technique for shortening the length of a drift region of a lateral MOSFET in a horizontal direction with respect to the substrate while retaining the high voltage resistance, by forming a trench in the surface of the drain drift region so as to obtain the drift length also in the direction of thickness of the substrate.
A method for manufacturing the high voltage lateral trench MOSFET is described below with reference to FIG. 17 appended hereto.
A trench 3 is formed in a surface layer of a P-type well 2 in a P-type substrate 1, and an N-type drain drift region 4 is formed around the outside of side and bottom surfaces of the trench 3. Then, the trench 3 is filled with an insulating film 5. Next, a P-type base region 8 and a P-type channel region 10 are formed by an ion implantation, and after forming a gate electrode 7, an N-type source region 9 and an N-type drain region 11 are formed by an ion implantation. Further, electrodes 14 and 15 are formed on the N-type source region 9 and the N-type drain region 11 respectively so as to complete the high voltage lateral trench MOSFET.
Patent Document 1: Japanese Patent Laid-Open Publication No. Hei 8 (1996)-97411